A conventional illumination device used in a liquid crystal display device or the like is disclosed in Patent Document 1. This illumination device is provided with a plurality of light sources and a light guide plate that guides light from the light sources that has entered the light guide plate and emits illumination light. The light sources are LEDs (light emitting diodes) and are arranged in one direction.
The light guide plate is resin-molded and has a substantially rectangular shape in a plan view, and has a light receiving face on a side face facing the light source and an illumination light emitting surface on a front surface thereof. On the rear surface of the light guide plate, a plurality of inclined faces that are inclined towards the light receiving face extend along the lengthwise direction of the light receiving face and are arranged in the optical axis direction of the light sources.
A plurality of prisms that extend along the optical axis direction of the light source are recessed into the light emitting surface, which is the front surface of the light guide plate. The prisms are arranged at a prescribed pitch in the lengthwise direction of the light receiving face, and a cross-sectional shape of the prisms parallel to the lengthwise direction of the light receiving face is formed in a trapezoid shape.
If the illumination device is driven, the light emitted from the light sources enters the light guide plate through the light receiving face. The light that entered the light guide plate is reflected by the light emitting surface and the rear surface of the light guide plate and is guided in the optical axis direction of the light sources. At this point, the light guided by the light guide plate and that has reached the prisms is diffused in the lengthwise direction of the light receiving face and reflected. Furthermore, the light guided by the light guide plate has a small incident angle at the light emitting surface due to reflection at the inclined faces of the rear surface, and therefore, as the light is repeatedly reflected, the light that is incident on the light emitting surface with a smaller incident angle than the critical angle is emitted. As a result, the illumination light is diffused in the optical axis direction of the light source and in the lengthwise direction of the light receiving face and is emitted from the light emitting surface.